1. Field of the Invention
This invention relates to the electrostatic speakers, and more particularly to electrostatic speakers which include a porous stator and are capable of full audio range performance.
2. Prior Art
Audio speakers typically fall within one of two categories: dynamic or magnetic driven devices and electrostatic speakers. Dynamic speakers rely on magnetic fields operating with respect to a moving cone and magnet that are driven by variable electromagnetic forces corresponding to the desired audio signal. Electrostatic speakers operate within much weaker, electrostatic force fields generated from a stationary stator which carries the audio signal and drives a conductive diaphragm suspended adjacent to the stator.
Electrostatic speakers have been available for decades; however, satisfactory high fidelity reproduction has been limited to very expensive systems, typically of large surface area. These limiting factors of high cost and cumbersome size have severely limited the consumer market for electrostatic speakers as part of a general sound reproduction system. This trend is contrasted by impressive advancements in dynamic speakers, both with reduction in cost and size. As a consequence, conventional dynamic speakers comprise 99% of the total domestic market. Electrostatic speakers constitute less than 1%.
The steady decline of cost of electronic components in other fields has not been matched by electrostatic design. To the contrary, these speakers remain extremely expensive. This is due in part to the large space requirement for electrostatic speakers. Because diaphragm displacement is extremely narrow, a large diaphragm is used to achieve an adequate displacement of air to develop desired amplitude, particularly at lower frequencies. In view of the required large diaphragm area, design and construction of drive systems and enclosures has tended to develop complexities in providing a uniform stator and corresponding diaphragm continuity.
One common element of electrostatic speakers is a rigid stator. The stator must be conductive to provide the variable voltage with attendant audio signal for driving the diaphragm. The rigidity of the stator is significant because the diaphragm must be maintained in a taut configuration to be fully responsive to the variations in electrostatic field strength carrying the audio signal. Any occurrence of nonuniformity in tension in the diaphragm may lead to nonlinear response in speaker output. Accordingly, the stator typically bears the stress of tension applied to the diaphragm.
Prior art stator elements have included rigid screens and grids, as well as perforated conductive plates. See, for example, U.S. Pat. No. 3,008,013 of Williamson et al and U.S. Pat. No. 3,892,927 of Lindenberg. Electrical contacts are provided on the stator for coupling leads from the voltage source. Perforations or open screen and grid structure enable passage of sound waves from the diaphragm to surrounding environment. This characteristic, referred to as acoustic transparency, imposes a significant limitation on the stator which conflicts with the need for uniform charge dispersion across the face of the stator. Uniform charge dispersion is favored because it provides continuity of force applied across the diaphragm. Lack of uniformity leads to reduction in efficiency in diaphragm response which limits audio output. Obviously, the ideal stator for charge distribution would comprise a flat plate without any form of opening or space interruption. This is impractical, however, because such a solid plate would block transmission of sound and defeat the purpose of the speaker.
Accordingly, the conflict between uniform charge dispersion and acoustic transparency arises with the need for open spaces or gaps in the stator to allow sound vibration to pass. These gaps constitute interruptions in the field continuity of charge distribution within the stator. In many prior art grid structures, such spacing was up to several centimeters in diameter. These large openings would clearly interrupt the uniformity of the electrostatic field. Preferred stators typically are formed of wire mesh having a woven matrix of conducting elements which have a continuously varying thickness, as well as grid openings in the several millimeter range. This configuration is illustrated in cross-section in FIG. 3 and represented in the disclosure of Rod in U.S. Pat. No. 3,345,469. It will be noted that large wire diameter is necessary to provide the strength to the grid needed for support of the diaphragm in tension. This size creates distance variations between the diaphragm and field source represented by h, h', h", etc. This difference is also a factor influenced by the opening size, which disturbs the uniformity of the field with increasing size.
Variations in openings sizes and shapes in stator plates is clearly shown in the various patents cited above. Such plates include molded or stamped perforations which range in dimensions up to several centimeters. Numerous complex configurations are illustrated for tensing or stretching the diaphragm across the stator to realize appropriate resonant frequencies needed for predictable sound reproduction.
Those skilled in the art will be familiar with other limitations within electrostatic speakers which have inhibited commercialization of systems which are cost competitive with conventional dynamic speakers. The previous discussion is simply for the purpose of demonstrating one particular area of technical difficulty which has challenged the electrostatic speaker industry. What is clear is that electrostatic speakers have been unable to keep pace with the continued expansive growth of dynamic speaker systems.